28th International Cosmic Ray Conference

2273

Upper Limit on the Diﬀuse Gamma Ray Flux Using Air
Shower Observations at Ooty
India-Japan GRAPES Collaboration:
Y. Hayashi,2 S.K. Gupta,1 Y. Ishida,2 N. Ito,2 S. Kawakami,1 H. Kojima,3D.K. Mohanty,1 P.K. Mohanty,1 T. Nonaka,2 A. Oshima,2 M. Sasano,2K. Sivaprasad,1 B.V. Sreekantan,1 S. Tamaki,2 H. Tanaka,2 S.C. Tonwar1
and T. Yoshikoshi1(1) Tata Institute of Fundamental Research, Mumbai 400005, India(2) Graduate School of Science, Osaka City University, Osaka 558-8585, Japan(3) Nagoya Women’s University, Nagoya 467-8610, JapanAbstract
We have attempted to determine diﬀuse gamma ray ﬂux using very large
area (560m2 ) and compact muon detectors operating with GRAPES-3 air shower
array. The showers with their cores incident within the EAS array but with noassociated penetrating track in the muon detectors are classiﬁed as muon-poor
showers. These are considered candidates for gamma ray initiated showers. We
have obtained upper limits on the ﬂux of isotropic gamma rays over the energy
region from 20 TeV to 500 TeV. A conservative upper limit on the ratio, Iγ /ICRof 3 × 10−4 at 90% C.L., over the energy region, 100 TeV to 500 TeV, has beenplaced.
1.

Introduction

Gamma rays coming from sources at cosmological distances, such as distant
AGN’s, and gamma rays due to cosmological origin are expected to arrive isotropically. In several tens of TeV region, the isotropic gamma ray ﬂux is expected
to be higher as a result of cascading from interactions of extremely high energy
cosmic rays with the cosmic microwave background radiation by the Bottom-Upmodel [4] or the Top-Down model[2]. Though, it is experimentally very diﬃcult
to distinguish gamma-rays from cosmic rays with ground based observations, upper limits have been placed by several groups which seem to question the validityof such models in the several tens of TeV to several tens of PeV energy range[1][2][3][5][7][8].

cpp. 2273–2276 2003
by Universal Academy Press, Inc.

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2.

Experiment

The GRAPES-3 air shower experiment is located at Ooty in southern
India (N 11.4, E 76.7 and 2200m altitude). It is observing two components of
air showers, the electro-magnetic component and the muon component, to study
cosmic gamma rays and particles in 10TeV to several tens of PeV energy range.
257 electron detectors (1m2 area plastic scintillator, 5cm thickness) are arranged
in a 8m span hexagonal dense array.
The muon detectors are arranged in the form of 16 well packed modules
with the cluster Total muon detection area is 560m2 consists of 232 proportionalcounters (cross-sectional area 10cm×10cm and length of 6m) arranged in 4 layers
separated by 15cm thick concrete layers under 2m thickness concrete absorber
giving 35m2 detection area for muons > 1GeV.
The triggering condition is any 10 electron detectors out of 120 detectors
at center of array. The trigger rate is about 13Hz.
3.

Data Analysis

We analyzed data observed from March 2000 to December 2001. The
electro-magnetic component data are used for estimation of primary cosmic rayenergy, core location and arival direction of air shower. The muon component data
are used to select candidate of gamma ray primary. We analyzed total 5.9 × 108events.
3.1. Electro-magnetic component
For the electro-magnetic component analysis, we used two stages of analysis method. First, we got total number of detected charged particles(Σnp (ob))
and estimated shower core location as center of gravity of highest most 7 scintillation detectors. For relatively large shower (Σnp (ob) > 70), we used maximum
likelihood method to estimate size, age and core location[10]. For small showers
NKG function ﬁtting is not suitable. Estimated shower size has too large error
due to detection ﬂuctuation. When we estimate primary particle energy, shower
size is better parameter compare with total number of detected charged particles
if size estimation is good. So we used shower size for primary particle energy
estimation Ne > 104 and used Ne(ob) for smaller shower. Under this condition
we selected following showers. (a) Air shower core should be inside a hexagonal
area as shown in ﬁg.1.. (b) Zenith angle < 25 degs.
3.2. Muon component
For the muon component analysis, the detected muon number has been
estimated from the observed muon tracks in the 4 layers of the detector whose2275Fig. 1.

GRAPES-3 Air Shower Array

direction must match with the direction of the air shower arrival direction determined from data on the electro-magnetic component. We use any 3 layers
coincidence out of 4 layers. We got the detection eﬃciency for any 3 layers outof 4 layers > 99% for penetrating particles using layer eﬃciency, > 98%. Estimation of number of muons is quite accurate especially for small number of detectedmuons. The number of detected muons depends on the air shower primary energy, distance from the core and the detection area. In the Ooty air shower array,typically several muons can be detected for total number of detected particles
100, distance from muon detector to core 40m. Average number of chance muon
is estimated as 0.07 per event. Muon lateral density distribution gives optimum
data collection area.
4.

Simulation and Calculation

Monte Carlo simulations with CORSIKA (Ver 5.62[6]) show that 2 to 3%
muons are expected from gamma ray induced air shower compared to the proton
induced showers for same primary energy. Since typical events observed by theOoty air shower array have 0 to several tens of muons, we have classiﬁed nomuon detected air showers as muon-poor events. We have calculated the upper
limit on the ratio of gamma ray ﬂux to all cosmic ray ﬂux using these muon-poor
events as the candidates for gamma ray induced showers. First, we estimatedmedian energy of primary cosmic ray for each detected particles bin. We did
not ﬁnd clear diﬀerence of detected number of particles between proton primaryand gamma-ray primary for 10 to a few 100 TeV energy range. Same time wegot number of muons for each shower and we estimated probability of muon lessshower by gamma primary for each distance bin from the center of muon detectors
for eﬃciency correction.

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5.

Discussion and Conclusion
Upper limit on the ratio of gamma ray ﬂux to the cosmic ray ﬂux is given

by:Iγ(90%C.L.) 11≤ Nmuonpoor
ICR
Nalleγ 1−nchance

where Nmuonpoor (90%C.L.) is a 90% conﬁdence level upper limit on the numberof muon-poor air showers assuming Poisson distribution, Nall is the total numberof air showers, eγ is the eﬃciency to detect gamma ray induced air showers asmuon-poor air showers, nchance is the average number of muons due to chancecoincidence. The result is shown in ﬁg.2. and compared with the upper limits
given by other groups. The Ooty result gives most strict upper limit for the 20 500 TeV region.

Fig. 2.

6.

Upper Limit of Isotropic Gamma Ray

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